Postimpressionism is a catchall label that has been applied to a number of artists whose work had impressionism at its roots. Many of these artists exhibited in the impressionist exhibitions, among them Paul Cézanne, Georges Seurat, and Paul Gauguin, but the postimpressionists gradually came to reject the impulse toward naturalism that had been a driving force behind impressionism. Cézanne replaced the irregularity of his impressionist brushwork with systematized notations that describe underlying form rather than momentary surface appearance. Seurat used a scientifically-based method of paint application (small touches of pigments in color sequences based on the latest theories of color and light) to explore the expressive qualities of the formal elements of painting. Similarly, Gauguin relied on the imaginative and evocative power of abstracted shapes and unmodulated color to express primitive and universal ideas. Pierre Bonnard and Édouard Vuillard were members of a group called the Nabis. Inspired by Gauguin, the Nabis insisted on the primacy of the physical existence of a painting -- paints arranged on a flat surface -- over the convention recognizing it as a re-creation of nature. In particular Bonnard and Vuillard were known for their small-scale paintings, for using areas of unmodulated colors in close harmonies, and for their quiet intimacy.